1. Field of Invention
This invention relates in general to power converters and, more specifically, to a dual half-bridge DC/DC converter and a dual full-bridge DC/DC converter with dual power transformers, PWM and Phase-Shift controls and capabilities of wide-range ZVS and zero circulating current.
2. Prior Art
In the field of power conversion, it is a common practice to convert electrical energy from one DC voltage level to the other isolated levels by using high frequency switching technology. The application of switching technology dramatically decreases the size of power converters and improves power conversion efficiency. While enjoying the benefits of switching technology, industry is facing new challenges of the further demands of higher power conversion efficiency and smaller sizes of converters, and lower Electromagnetic Inference (EMI) emission, which is caused by switched currents and voltages.
In order to improve converter efficiency, reduce the size of converters and minimize EMI, tremendous effort has been made on four areas: 1. Wide-range Zero Voltage Switching (ZVS), 2. Circulating current elimination, 3. Recovery of the energy resulted from the reverse recovery of output rectifier devices, and 4. Output rectifiers' voltage ringing elimination or clamping; and many topologies have been well developed, which include full-bridge converters with phase-shift control or Pulse Width Modulation (PWM) control, asymmetrical half-bridge converters, etc. One of the most popular topologies is the phase-shifted full-bridge DC/DC converter, especially for high power applications. Such a circuit is described in detail in a Texas Instruments' application note U-136A, entitled “Phase Shifted Zero Voltage Transition Design Consideration and the UC3875 PWM Controller”, published in May 1997. The phase-shifted full-bridge DC/DC converter herein described relies on the primary current including power transformer's magnetizing current and the current reflected from the secondary to charge or discharge the parasitic capacitive elements of switching devices of the bridge's lagging leg, and the circulating current and its corresponding energy stored in the transformer leakage inductance to energize a resonation between the capacitive elements of the leading leg's switching devices and the leakage inductance. When the secondary output current reaches a certain level, the switching devices' parasitic capacitance can be fully charged or discharged during the dead time of gate signals. The switching devices can be such controlled that they are turned on after the voltages across the devices decrease to zero. This switching technique is the so-called Zero Voltage Switching. Since there is no capacitance energy discharged into the turning-on devices, such a control essentially eliminates the switching loss in certain heavy load range. But in the rest load range, the switches have to be turned on with certain voltage across the devices, and the energy stored in the parasitic capacitive elements eventually dissipates into the switching devices. This non-zero voltage switching results in part of switching losses and more EMI emission. At light load, especially zero load, effective switching duty cycle and the corresponding voltage-second applying to the power transformer usually reduce when the current in the output filter inductor gets into discontinued conduction mode. In the scenario, neither the primary current reflected from the secondary nor the magnetizing current can substantially reduce the voltage across the switches to be turned on and the switching could result in a significant power loss. Moreover, the topology did little to recycle the leakage inductance energy caused by the reverse recovery of output rectifier diodes, and to clamp the voltage ringing resulted from the free-running leakage inductance energy at both the primary and secondary. Eventually, part of the leakage inductance energy is dissipated in the form of heat inside the power train and the rest is radiated into space in the form of electric and magnetic fields. It could herefrom cause both thermal and EMI problems.
To mitigate the problems above, a simple but effective clamping circuit was invented by Richard Redl, et al, and detailed in U.S. Pat. No. 5,198,969. The node connecting one end of a power transformer primary winding and one end of a resonance inductor is clamped to positive and negative terminals of a DC input by two cascaded diodes. The clamping circuit with the resonance inductor reduces the transient current caused by the reserve recovery of output rectifiers, captures most of the transient energy on its primary side, and minimizes the voltage ringing on both the primary side and secondary side significantly. The captured energy in the form of a current flowing through the resonance inductor circulates inside the loop comprising the inductor, one of the clamping diodes and one switch. While circulating inside the loop, part of the energy is dissipated because of the voltage drop among the devices. The rest energy, if any, can be utilized for resonation and recycled back to the DC source when the switch shorting the loop is turned off. At the cost of duty cycle loss, the transient current can be minimized by using a larger resonance inductance to reduce the current slew rate at the output rectifiers, and circuit efficiency, to some degree, could be increased. The use of a larger resonance inductance can also extend ZVS to a lighter load.
To further extend ZVS range, a lot of circuits have also been invented. The circuits can be categorized into two types: one is an active switch-controlled resonance network. The auxiliary switch is usually turned on at zero current. It activates a resonation to create a zero voltage condition for main switches to be turned on. The other is simply a LC network connecting to the bridge switches. The network produces a load-independent resonant current, which helps the bridge switches to achieve ZVS in a wide range of loads. One of the examples is the circuit invented by Pradeep Madhay Bhagwat, and the related United States patent is U.S. Pat. No. 5,875,103, entitled “Full Range Soft-Switching DC-DC Converter”. These circuits indeed increase the ZVS range. However they didn't address the other problems mentioned above.
A load-dependent circulating current is one of the major drawbacks of the existing ZVS full-bridge DC/DC converters. The circulating current passes through most of the power train of the full-bridge DC/DC converter, including two bridge switches, a resonance inductor, if any, power transformer primary and secondary windings and output rectifiers, when both top or bottom switches are turned on. During this period, no energy is transferred from the primary side to the secondary side. This circulating current causes a substantial power loss. The circuit, introduced in the U.S. Pat. No. 5,946,200 with a title of “Circulating Current Free Type High Frequency Soft Switching Pulse Width Modulated Full Bridge DC/DC Converter”, eliminates the circulating current by using a resonance network on the secondary side to completely draw the circulating energy and herewith the circulating current out of the power train and discharge it to its DC output. Therefore, it minimizes the power loss related to the circulating current. However, the resonance network also removes the necessary circulating energy to accomplish ZVS, and its leading leg switches have to turn on with a near full DC bus voltage across their parasitic capacitances, which will result in some power losses and could cause EMI problems. Because of this reason, some switching devices with high parasitic capacitance, like MOSFETs (metal-oxide silicon field-effect transistors), may not be suitable for this topology. Another disadvantage is that, compared with the full-bridge DC/DC converter with clamping diodes invented by Richard Redl, et al., this circuit has a much higher reverse voltage across the output rectifiers, especially at startup, because of the resonation of the resonance network.
Another way to eliminate the circulating current is to use asymmetrical control. The topologies which can utilize such a control include an asymmetrical half-bridge DC/DC converter and an asymmetrical full-bridge DC/DC converter. An application of asymmetrical half-bridge DC/DC converters was detailed in U.S. Pat. No. 6,496,396 “Reverse Recovery Circuit, Method of Operation Thereof And Asymmetrical Half-bridge Power Converter”. Asymmetrical bridge DC/DC converters could work quite well under some conditions. The conditions include low input variation, low output DC voltage regulation range and small or slow step load changes. If the conditions can not be met, the circuit could get into a severe asymmetrical mode, where soft switching could lose and the current stress of the bridge switches and voltage stress of the output rectifiers increase. Furthermore, since the primary current passes through the bridge capacitor(s), the capacitance value needs to be relatively large, and therefore the capacitor's voltage may not change fast enough to follow PWM duty cycle's change during a large step load response. A large step load could easily cause power transformer saturation. Because of this reason, asymmetrical bridge DC/DC converters are not suitable for high power applications and their bandwidths are usually lower than those of conventional bridge DC/DC converters.
Although these efforts of the previous arts, to various degrees, mitigate power loss and decrease component stress, there was no successful story of handling all the mentioned problems with one single topology yet. It would such be desirable for industry to have a converter which has a wide-range ZVS capability, always operates at maximum duty cycle, fully utilizes magnetic components, minimizes or eliminates circulating current, recovers reverse recovery energy and well clamps or eliminates the voltage ringing at output rectifiers. This presented invention is going to disclose a new control method and a novel topology in the following sections; by applying the proposed control to the topology, a new family of DC/DC converters were created. The converters are able to operate at a constant 50% duty cycle, like an open loop Bus Converter, while regulating its output voltage, to achieve most, if not all, desirous features listed above and improve overall circuit efficiency and performance to a new level.